. >> reporter: defense attorneys for george zimmerman are attacking prosecution assertions that trayvon martin is the one yelling for help on that 911 call, the night he was shot and killed. susan mcginnis reports from the courthouse in sanford, florida. >> reporter: the defense called several friends of george zimmerman. each one took the stand. and each said they believed it's his voice on this 911 call, screaming for help. [ screaming ] >> so you think he's yelling help? >> yes. >> chose voice is this? >> george zimmerman's voice. >> definitely georgey's. >> no doubt in my mind that is george zimmerman. >> reporter: on friday, zimmerman's mother and uncle testified the screams were zimmerman's. the defense claims the neighborhood watchman shot trayvon martin in self-defense after the teen attacked him. but the family of martin believes it was his. the case >> reporter: the case could turn on the deadsly screams. >> reporter: during cross- examination, prosecutors pointed out that sondra osterman wrote a book about the case, with the proceeds going to george zimmerman. they spent thou